This document claims priority and contains subject matter related to Japanese Patent Application Nos. 2000-321397 filed on Oct. 20, 2000 and 2001-273280 filed on Sep. 10, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a developer as well as an image forming apparatus and method using the developer.
2. Discussion of the Background
As conventional latent image developing methods using a toner, two-component developing methods represented by a magnetic brush developing method disclosed in U.S. Pat. No. 2,874,063 and one-component developing methods are known.
In a dry type two-component developer used for a two-component developing method, fine toner particles are retained on the surface of relatively larger carrier particles by static electricity caused by friction between both particles. When the toner particles come close to a latent image, the toner particles are attracted to the latent image and the latent image is visualized, because the electric field strength of the latent image attracting the toner particles is larger than the binding strength between the toner particles and the carrier particles. Thus, the developer is repeatedly used, refilling the toner consumed for the development.
Therefore, the mixing ratio of the carrier and the toner, (i.e., the toner concentration) should be fixed to form a stable image density in the two-component developing method. Accordingly, a toner supplying mechanism and a toner concentration sensor are required for the developing device, which increases the size of the device and makes the printing operation more complicated.
On the other hand, in a one-component developing method, static electricity caused by friction between a toner and a developing sleeve of the developing device or a magnetic attraction between the toner""s magnetic particles and the developing sleeve""s magnet retains the toner on the developing sleeve. When the toner particles come close to a latent image, the toner particles are attracted to the latent image because the electric field strength of the latent image attracting the toner particles is larger than the binding strength between the toner particles and the developing sleeve.
Therefore, the one-component developing method is advantageous because the toner concentration does not need to be controlled. Thus, the size of the developing device can be reduced. However, it is difficult to apply the one-component developing method to a high-speed copier because the concentration of the toner particles in the developing area is smaller than that of the two-component developer and the developed volume of the toner on a photoreceptor is not enough.
Further, even in the two-component developing method, when the toner is not charged enough because the linear velocity of the developing sleeve is fast in a high-speed copier, the toner on the developer tends to leave the carrier, resulting in toner scattering. Therefore, the magnetic two-component developer including the magnetic toner is used even in the two-component developing method.
However, when the magnetic toner is used for the two-component developer, the toner magnetization becomes large if the volume of the magnetic particles is increased, resulting in deterioration of the developing capability in the two-component developing method. Further, when the volume of the magnetic particles is decreased, a reddish image without enough density is produced. To improve the drawback, when a non-magnetic black pigment such as carbon black is used, the chargeability of the toner deteriorates and background fouling tends to occur.
Accordingly, an object of the present invention is to provide a two-component developer which sufficiently charges a toner and forms a quality image without toner scattering and background fouling.
To achieve these and other objects, the present invention provides a two-component developer including a magnetic toner including magnetic particles coated with carbon black, and a magnetic carrier configured to carry the magnetic toner on a surface thereof.